Pretty much every organization out there that is working on web lead projects uses Slack as a preferred mode to communicate between the teams. Not only is it extremely easy to collaborate on Slack, but the fact that it generally trumps IMs to newer features is what it makes really useful to be on Slack. Features like responding to individual messages with emoticons or even editing and already sent text messages, are something that the likes of WhastApp do not allow you. Oh, and how can you forget the bots and GIF support.
Until now, one feature that Slack did not have was the ability to drop voice notes or call up those in your team. A couple of days ago, Slack announced via their blog that they would be enabling calling across all the Slack services, be it on mobile, desktop or the web app. This is a phased rollout and although we have not received it yet on our Slack channel, it should not be too long before it does happen. When you do get the feature, you will see a call icon right next to the gear icon that is available on every individual chat screen.
If you are on a free Slack plan, you will only be able to make a direct phone call to individuals. However, for the premium users, you can initiate group calls and even respond within the call by using emoticons, so that those in the call can have your silent approval or disapproval. This move would definitely take productivity to another level as you would not need to pick up your smartphone to dial in to a team member, just call him on Slack when sitting on your work desk and get the job done.